Landing approach aids

ABSTRACT

THE INVENTION RELATES TO AN AIRBORNE LANDING APPROACH AID FOR AN AIRCRAFT IN WHICH A SYMBOL PARALLEL TO AND DEPRESSED WITH RESPECT TO THE HORIZON IS PRESENTED ON A HEADUP DISPLAY. WHEN THE AIRCRAFT IS REMOTE FROM THE TOUCHDOWN POINT OF A RUNWAY THE LINE IS DEPRESSED BY THE GLIDE SCOPE ANGLE, NORMALLY 3*, WITH RESPECT TO THE HORIZON. TO ALLOW FOR ERRORS INTRODUCED BY THE DISTANCE BETWEEN THE PILOT AND THE MAIN LANDING GEAR OF THE AIRCRAFT THE ANGLE OF DEPRESSION OF THE SYMBOL VARIES WITH DISTANCE FROM THE TOUCHDOWN POINT OF RUNWAY SO THAT IT IS THE LANDING GEAR, AND NOT THE PILOT&#39;&#39;S EYES, WHICH TRACK DOWN THE GLIDE SCOPE.

Jan. 19, 1971 G. R. SLEIGHT 3,555,397

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United States Patent 3,555,897 LANDING APPROACH AIDS George RobinSleight, Coxheath, near Maidstone, Kent, England, assignor to ElliottBrothers (London) Limited, London, England, a British company Filed June27, 1969, Ser. No. 837,170 Claims priority, application Great Britain,June 29, 1968, 31,189/ 68 Int. Cl. G01c 21/06 U.S. Cl. 73-178 2 ClaimsABSTRACT OF THE DISCLOSURE The invention relates to an airborne landingapproach aid for an aircraft in which a symbol parallel to and depressedwith respect to the horizon is presented on a headup display. When theaircraft is remote from the touchdown point of a runway the line isdepressed by the glide scope angle, normally 3, with respect to thehorizon. To allow for errors introduced by the distance between thepilot and the main landing gear of the aircraft the angle of depressionof the symbol varies with distance from the touchdown point of therunway so that it is the landing gear, and not the pilots eyes, whichtrack down the glide scope.

This invention relates to a landing approach aid for aircraft.

It has been suggested that the simple display, on a headup display, of ahorizon line depressed by the angle of the desired glide slope from thehorizontal should form a useful approach aid for aircraft whenperforming a landing manoeuvre.

The primary virtue of such a system is that it does not depend on theavailability or correct functioning of an I.L.S. system.

Although the feasibility of such a scheme has been demonstrated by theBlind Landing Experimental Unit at Bedford, the latter scheme hascertain limitations,

Firstly, with a large pilots eye to landing wheel distance aconsiderable inaccuracy can occur during a landing approach, between theprojected flight path as seen by the pilot and the flight path asexperienced by the main wheels. If the pilot tries to hold a depressedhorizon line without correction on the desired touchdown point then themain wheels would touch the runway well short of the point. Secondly,the touchdown point (glide slope origin) is not well defined at allairfields and a pilot may well have trouble in holding the depressedsight line on an ill-defined point.

This invention is concerned with a system to display .accuratelypositioned touchdown zone and threshold markers which are corrected forthe errors due to pilot displacement from the main wheels.

According to the invention a landing approah aid for an aircraftcomprises:

a head-up display means operable to develop a signal representing pitchattitude of the aircraft;

means operable to develop a signal representing aircraft altitude;

means operable to develop constant signals defining the location of themain landing gear of the aircraft with respect to the pilot;

means operable to develop a constant signal representing a desired glideslope angle; and

circuitry responsive to the signals from the various means referred toabove so as to develop an output signal 0perable to control the positionof a target marker symbol on the head-up display so that the pilot byflying the the aircraft to maintain the symbol in coincidence with adesired point on a runway corresponding to the origin of the slideslope, ensures that it is the aircrafts main landing gear (and not thepilots eye) which tracks along the glide slope.

The landing approach aid may include means operable to develop a signaldefining the location of the runway threhold with respect to the glideslope origin; and circuitry responsive to the signals from the lattermeans and to the various means referred to in the least precedingparagraph so as to develop an output signal operable to control thedisplay so as to produce on the display, a symbol which coincides withthe runway threshold.

The invention is hereinafter described with reference to theaccompanying drawings, in which:

FIG. 1 is a diagram, grossly distored as to angles and dimensions,illustrating certain essential features of the invention;

FIG. 2 is a graph showing the variation from a nominal three degreeslide slope with approach to touchdown;

FIG. 3 is a schematic diagram showing an implementation of theinvention; and

FIG. 4 is a diagram showing runway and symbols which, in accordance withthe invention, may be displayed on the head-up display.

Referring to FIG. 1, if the landing approach is to be made along a glideslope having an angle 7 and a display is derived from an attitudereference and depressed by an angle A6 below the horizontal, then ifA027 and if the aircraft is small and on the correct slide slope, thanthe projected position of the line A0 will lie on the glide slopeorigin.

By displaying a depressed horizon line, Le. a horizontal line depressedbelow the horizon by an angle equal to the desired approach path anglethen, neglecting the effect of aircraft dimensions, the pilot canmonitor his approach by the relation between the depressed horizon andthe touchdown point. Additionally, at an early-stage in the approach,say as seen from a position on the glide slope well above 2,000 ft., theposition of the runway threshold and touchdown points are effectivelycoincident.

If now we consider the effect of aircraft dimensions then, referring toFIG. 1 and assuming the aicraft to be flying along a course verticallyabove the runway centre line, the wheel distance to touchdown is:

S (1 radian=57.3)

.where 0 is the angle between the runway centre line and a line drawnfrom the pilots eyes to the glide slope origin.

If 7 3 then h-l-Ah 01=3 i 19.1

3 Similarly: where is the angled subtended between the centre line ofthe runway and a line drawn from the pilots eyes to the runwaythreshold.

Assuming S to be the agreed ICAO standard, 1,000 ft., and, as typicalvalues for Ah and S we assume the values 40 ft. and 100 ft, respectivelythen:

Clearly, as h tends to infinity then 0 and 6 tend to 3".

Otherwise expressed at a high altitude on the approach, the error due todisplacements of the pilots eye from the main wheels is negligible.Below about 1,000 ft., however, the error becomes significant. FIG. 2shows the values of 6 and 0 plotted against height in order to ensurethat the main wheels track along a 3 glide slope (for a particular caseAh=40 ft. and S 100 ft.).

As can be seen, even the depressed touchdown line actually requires tobe depressed by 3.7 rather than 3 at 200 ft. to maintain the aircraftwheels on the correct path. If this correction is not applied to thedepression angle then there is a danger that the pilot may push thealrcraft nose down to maintain an uncorrected 3 line at the touchdownpoint and the aircraft would then land short of the touchdown correctpoint and with a higher rate of descent.

The values for Ah (40 ft.) and S (100 ft.) employed in the aboveequations approximate to those for Concorde aircraft.

In the latter aircraft the eye to ground height with aircraft nose wheelon the ground is 27 ft. and the cockpit is 100 ft. ahead of the rearmostmain gear. Assuming, for Concorde, a pitch attitude of 12 on approach toland then, using the nomenclature of FIG. 2:

From the above it will be seen that the figures in the equations are notunrepresentative of large aircraft currently under development.

Referring to FIG. 3 it will be seen that the signals 0 and!) forcontrolling the depressed horizon and runway threshold markers,respectively, of the head-up display (FIG. 4) are derived from a radioaltimeter 11, a vertical reference unit 13, a signal source 15 which isoperable to develop a signal representing the constant S means 17operable to develop from the signal S a constant signal a signal source19 operable to develop from the signal S a signal representing theconstant specifically the constant and an amplifier 21 having a gain 'yspecifically 'y=3, the guide slope angle.

The vertical reference 13 develops an output signal 0 which is appliedto a multiplier 23 which also receives a signal S from the source 15 andfrom these signals produces an output signal representing the parameterAh.

The output signal Ah from the multiplier 23 is applied to one input ofthe amplifier 21 which receives, at a second input, a signal h from theradio altimeter 11 and respectively. The device 27 develops an outputsignal the device 29 develops an output signal The output signal 3(h+Ah)from the amplifier 21 is applied to a divider 31 which also receives asignal from the subtractor 27 and develops the output signal 0 Theoutput signal 0 from the divider and a signal 'y(=3) from a source 20are applied to a substractor 33 which develops an output signal 0correction, i.e. (0 3) The signal 7(=3) from the source 20 is applied toa subtractor 35 which also receives a signal 0 from the verticalreference 13 and from these signals produces an output signal (0-3). Thelatter signal is applied, together with a signal 0 correction, to afurther subtractor 37 which, develops the output signal 0 (=0 -0).

In like manner the output signal (3(h+Ah) from the amplifier 21 to asecond divider 39 to which is also applied the signal developed by thesumming amplifier 29.

In response to the signals the divider 39 develops the output signal 0The latter signal is applied to a subtractor 41 which receives thesignal 'y(=3) from the source 20 and develops the output signal(icorrecuon i.e. (R 3). The latter signal is applied, together with asignal (0-3) from the subtractor 35 to a further subtractor 43 which, inresponse, develops the signal 6 '=0 0. Signals 0 and 0 are employed tocontrol the position, on the display, of the depressed horizon line andthe threshold marker symbol, respectively (FIG. 4).

In the latter figure, in addition to the symbols representing the trueand depressed horizons and the threshold marker, there is a flight pathvector symbol. When the aircraft is not on the glide slope or has anundesired attitude, the latter symbols is displaced with respect to thedepressed horizon symbol; the pilots task is to manoeuvre the aircraftso as to bring about the alignment of symbols, as illustrated.

The latter symbols are formed by a waveform generator (not shown) of thedisplay in response to the signals 0 and 0 from the subtractors 37 and43 respectively. The depressed horizon and threshold marker symbols areformed on the target of a cathode ray tube of the head-up display by adiscrete line writing (as distinct from brightup of appropriate segmentsof a television type raster) on the target. The cathode ray tube beamdeflection circuitry (not shown) for producing symbols on the target ofthe cathode ray tube, may be of a conventional character and,accordingly, is not discussed here.

I claim:

1. A landing approach aid for an aircraft which comprises:

a head-up display;

means operable to develop a signal representing pitch attitude of theaircraft;

means operable to develop a signal representing aircraft altitude;

means operable to develop constant signals defining the location of themain landing gear of the aircraft with respect to the pilot;

means operable to develop a constant signal representing a desired glideslope angle; and

circuitry responsive to the signals from the various means referred toabove so as to develop an output signal operable to control the positionof a target marker symbol on the head-up display so that the pilot byflying the aircraft to maintain the symbol in coincidence with a desiredpoint of a runway corresponding to the origin of the glide slope,ensures that it is the aircrafts main landing gear (and not the pilotseye) which tracks along the guide slope.

2. A landing approach aid according to claim 1 and including meansoperable to develop a signal defining the location of the runwaythreshold with respect to the glide slope origin; and circuitryresponsive to the signals from the latter means and to the meansreferred to in claim 1 so as to develop an output signal operable tocontrol the display so as to produce on the display, a symbol whichcoincides with the runway threshold.

References Cited UNITED STATES PATENTS 3,355,941 12/1967 Birmingham73178 DONALD O. WOODIEL, Primary Examiner

